The present invention relates generally to liquid pumping systems, wherein one liquid is pumped or fed into the stream of another liquid. More particularly, the present invention relates to a liquid pump with a liquid reservoir and modified pressure relief slot to minimize leaking.
There are situations in which it is necessary to inject or feed one liquid into the stream of another liquid. Some liquid pumping systems require an occasional injection of liquid while others need a more continuous feed of the liquid. Still others might require a combination of the two. For purposes of this disclosure, it is understood that the term “feed” will include inject.
One such common application is in the field of water treatment wherein certain chemicals, such as chlorinating solutions, fluorination chemicals and other liquids, are fed into the water stream at a point prior to its delivery for end use by consumers. It is important to maintain certain percentage levels of these added liquids in order to assure adequate functionality without exceeding predetermined concentrations which could be objectionable or even harmful to the consumer.
A variety of apparatus is available in the industry to perform this chemical feed task. Such apparatus typically takes the form of a pump, wherein pump speed and chemical feed rate is controlled by well known electronic means which employs chemical concentration detection means and provides voltage or current signal output for use by the pump drive system to adjust its feed rate. This system operates in a closed loop fashion to maintain a relatively stable concentration of the desired chemical in the water stream.
Pumps used to inject chlorinating solutions, such as Sodium Hypochlorite (NaOCl), into a pressurized water stream frequently encounter problems associated with crystallization of the NaOCl. Although crystallization, with its tendency to lock parts, has been previously considered in various pump designs, the abrasive nature of these crystals was not thoroughly considered.
Positive displacement pumps having a ceramic piston and a liner are often plagued with consequential problems arising from such abrasive crystals. During normal pump operation, the piston will rotate and reciprocate in and out of the pump head. Upon outward movement of the piston, suitably designed sealing elements will wipe the piston surface to minimize dragging of any pumped liquid out of the pump head. This squeegee action of the seals is not, however, perfect. Some liquid is always present as a film on the exposed piston surface.
This primary difficulty occurs most often in those installations where the NaOCl injection pump does not run continuously. In such applications, the pump might run for as little as one (1) hour and then be allowed to sit idle for the next twenty-three (23) hours. If the piston is partially or fully withdrawn from its mating pump head during such idle time, the previously described NaOCl film will dry, resulting in hard, abrasive crystals forming on the piston surface. At this point, the piston surface can be likened to a nail file with a fine abrasive.
When the pump next begins to run, the piston having the newly formed abrasive surface will travel past the seal elements on its way into the pump head. This has been found to prematurely wear the seal elements such that they gradually lose the ability to perform their squeegee action on the piston. This in turn leads to an increase in crystallization during idle time and ultimate failure of the seal.
Once seals have been sufficiently worn, additional problems arise during idle time. NaOCl injection pumps of the type being addressed typically utilize a slight negative pressure of approximately 1-2 psig on the inlet port to preclude leakage of NaOCl out of the pump head during idle times. Pumps of the prior art typically include a pressure relief slot, also known as a “scavenger slot,” to provide for such negative pressure. However, the combination of a worn seal with a pressure relief slot allows the negative pressure to aspirate air into the pump head. This air flow will gradually lead to evaporation of NaOCl liquid within the pump head such that crystallization will cause the piston to lock and be unmovable when the pump is later energized.
Design of the pump drive mechanism can be such as to assure full piston insertion into the pump head during idle time but such mechanisms add considerably to complexity, size and cost.
Therefore, it would be desirable to provide an effective solution to the crystallization problems described above, with minimum cost and without increasing size or complexity of the pump. More particularly, it would be desirable to provide a simply designed pump with provisions for reducing crystallization caused by evaporation of such chemicals as sodium hypochlorite, together with further provisions for minimizing leakage.